Do you mean transferable HK 21's?
There are plenty of semi-auto 21's out there. You can buy one new from ORF, Michael's Machines or a few other places...
I have always wanted one, but have never justified the price. A 21 and a semi M2.
To my knowledge ALL of the semi 21's "in the states" were made (converted) right here in the good ol' USA. Initially by F.J.Vollmer & Co. with others to follow.
Because I don't follow them I'm not quite sure, but I would expect to pay close to 5 figures for a NICE semi 21.
I've shot a few RR 21's (not the 'e') and they will flat beat the crap out of you, and sooner or later, itself.
Semi Auto HK 21 are selling for around 8 to 10K now. Mostly because the parts are hard to find and your need a very good smith to build you one.
And a few more have built or still make HK21s. There is a guy on HKpro who has been building a HK 21, 51B and G3K B seems like every other week.
I too would love to own one, but having never shot one semi or FA I would like to know if they are really worth it.
When the .308 C-Mag is out the HK 11 might be sought out more than the belt fed 21.